Proton irradiation of plastic scintillator bars for POLAR-2

Abstract

POLAR-2, a plastic scintillator based Compton polarimeter, is currently under development and planned for a launch to the China Space Station in 2025. It is intended to shed a new light on our understanding of Gamma-Ray Bursts by performing high precision polarization measurements of their prompt emission. The instrument will be orbiting at an average altitude of 383 km with an inclination of 42° and will be subject to background radiation from cosmic rays and solar events. In this work, we tested the performance of plastic scintillation bars, EJ-200 and EJ-248M from Eljen Technology, under space-like conditions, that were chosen as possible candidates for POLAR-2. Both scintillator types were irradiated with 58 MeV protons at several doses from 1.89 Gy(corresponding to about 13 years in space for POLAR-2) up to 18.7 Gy, that goes far beyond the expected POLAR-2 life time. Their respective properties, expressed in terms of light yield, emission and absorption spectra, and activation analysis due to proton irradiation are discussed. Scintillators activation analyses showed a dominant contribution of β + decay with a typical for this process gamma-ray energy line of 511 keV.

Keywords: Cosmic rays; POLAR-2; Plastic scintillator; Protons; Radiation.

Fig. 1

Left—Compton scattering of an incoming $\gamma$-ray between two scintillator bars [3]. Right—Assembled POLAR-2 target made of sixty four EJ-248M bars wrapped in 3M Vikuiti and Toray Claryl reflective foils

Fig. 2

Two EJ-200 plastic scintillators. Left—scintillator bar chosen for POLAR-2. Right—cylindrical plastic as our reference point (see details in the text)

Fig. 3

Scintillators dose rate distribution among the 100 POLAR-2 polarimeter modules. The white marker on the colorbar shows the dose rate averaged over the full instrument

Fig. 4

Experimental set-up for radioluminescence emission of plastic bars

Fig. 5

Comparison of radioluminescence emission spectrum of plastic bars. Left—three RL spectra for EJ-200 measured before irradiation. Right—averaged after three samples for each type of scintillator RL spectra for EJ-200 and EJ-248M before irradiation

Fig. 6

Comparison of radioluminescence emission spectrum of plastic bars. Left—RL spectra for EJ-200 measured after irradiation for samples 1–4. Right—RL spectra for EJ-248M measured after irradiation for samples 1–4. Black lines show averaged signals before irradiation (see Fig. 5)

Fig. 7

Experimental set-up for absorption measurements, where the light was transmitted along scintillator length

Fig. 8

Absorbance measured for irradiated sampled of EJ-200 (left) and EJ-248M (right) scintillators. Solid lines correspond to the cases, where measured was done along short scintillator edge (5.9 mm) and dashed lines—along the longest scintillator edge

Fig. 9

Experimental set-up scheme. Left—PMT, scintillator and collimator configuration. Right—analog electronic readout scheme

Fig. 10

Example of dark count spectrum measured for PMT XP2020Q with single photo-electron peak. Red curve shows the Gaussian fit used to determine the peak position

Fig. 11

Energy spectrum of ¹³⁷Cs measured for EJ-200 plastic bar: left—as a function of distance from PMT window, right—the position of Compton edge (blue dashed line) determined by the fitting Eq. 3 (red line)

Fig. 12

Light yield of EJ-200 (top) and EJ-248M (bottom) expressed in photo-electron units as a function of distance from PMT window for non-irradiated and irradiated samples. The light yield change normalization point was chosen as a value for non-irradiated scintillator at distance of 11 mm

Fig. 13

Gamma-ray energy spectrum measured with HPGe detectors after 58 MeV proton scintillators irradiation. Black lines show experimental data. Red lines present measured background normalized to the same live time for each HPGe detector. Dashed blue lines show the identified 511 keV peaks. The pile-up of this line was also observed at 1022 keV for one detector

Fig. 14

Decay time distributions of the 511 keV line (black points) measured for 58 MeV proton irradiated plastic scintillators with fitted exponential curves (red lines). The experimental uncertainties are smaller than the bullet marker size